Detrimental effects of reloading recovery on force, shortening velocity, and power of soleus muscles from hindlimb-unloaded rats

To better understand how atrophied muscles recover from prolonged nonweight-bearing, we studied soleus muscles (in vitro at optimal length) from female rats subjected to normal weight bearing (WB), 15 days of hindlimb unloading (HU), or 15 days HU followed by 9 days of weight bearing reloading (HU-R). HU reduced peak tetanic force (Po), increased maximal shortening velocity (Vmax), and lowered peak power/muscle volume. Nine days of reloading failed to improve Po, while depressing Vmax and intrinsic power below WB levels. These functional changes appeared intracellular in origin as HU- induced reductions in soleus mass, fiber cross-sectional area, and physiological cross-sectional area were partially or completely restored by reloading. We calculated that HU- induced reductions in soleus fiber length were of sufficient magnitude to overextend sarcomeres onto the descending limb of their length-tension relationship upon the resumption of WB activity. In conclusion, the force, shortening velocity, and power deficits observed after 9 days of reloading are consistent with contraction-induced damage to the soleus. HU- induced reductions in fiber length indicate that sarcomere hyperextension upon the resumption of weight-bearing activity may be an important mechanism underlying this response.